We will investigate the use of magnetic resonance imaging (MRI) to improve the local staging of breast carcinoma through the enhanced detection of additional mammographically occult foci of carcinoma, and thereby, to optimize treatment selection. The patients we will study are women with newly diagnosed breast cancer following initial excisional biopsy with positive margins of resection, who are being considered for re-excisional biopsy and breast conservation therapy. After excisional biopsy, residual tumor is left in the breast in 32-63 percent of cases. Detecting the residual disease and determining its extent are important for further treatment planning. Assessment of the completeness of surgical excision, however, can be very difficult. Neither positive or negative margins of resection are reliable indicators of the completeness of tumor removal. Mammographic evaluation including specimen radiography and post-biopsy mammography following excisional biopsy is relatively insensitive for the identification of residual tumor. We and several other investigators have demonstrated that MRI can detect multifocal and diffuse carcinoma in patients presumed to have unifocal or localized disease on the basis of mammographic and clinical findings. We hypothesize that the detection of multifocal or diffuse carcinoma, unsuspected by mammography, physical examination, or pathologic findings will result in improved patient selection for breast conservation therapy or mastectomy. Patients who are being evaluated for possible re-excisional biopsy and breast conservation therapy will be examined with MRI prior to definitive surgery. If additional suspicious areas are identified on MRI, these will be biopsied with MRI guidance. Pathologic confirmation of diffuse or multifocal malignant lesions will result in re-evaluation of conservative treatment in these patients. Correlation of MRI, mammographic, clinical, and pathological findings will generate a profile of patients at risk for occult, diffuse or multifocal disease who should undergo MRI prior to treatment recommendation.